1. Field of the Invention
The present invention generally relates to the fabrication of integrated circuits, and, more particularly, to the formation of inter-level conductive connections and a corresponding monitoring of this process in semiconductor devices comprising one or more metallization layers.
2. Description of the Related Art
In an integrated circuit, a large number of circuit elements, such as transistors, capacitors and the like, are formed in or on an appropriate substrate, usually in a substantially planar configuration. Due to the large number of circuit elements and the required complex layout of many modern integrated circuits, generally the electrical connection of the individual circuit elements may not be established within the same level on which the circuit elements are manufactured, but such electrical connections may be established in one or more additional “wiring” layers, also referred to as metallization layers. These metallization layers generally include metal lines, providing the inner-level electrical connection, and also include a plurality of inter-level connections, also referred to as vias, wherein the metal lines and vias may also be commonly referred to as interconnections. In this specification, unless otherwise specified, a contact connecting to a circuit element or a portion thereof, for example a gate electrode or a drain or source region of a transistor, may also be considered as an inter-level connection.
Due to the continuous shrinkage of the feature sizes of circuit elements in modern integrated circuits, the number of circuit elements for a given chip area, that is, the packing density, also increases. The increased packing density usually requires an even greater increase in the number of electrical interconnections to provide the desired circuit functionality. Therefore, the number of stacked metallization layers may increase as the number of circuit elements per chip area becomes larger. The fabrication of a plurality of metallization layers involves extremely challenging issues to be solved. For example, mechanical, thermal and electrical reliability issues of up to twelve stacked metallization layers that are required for sophisticated aluminum-based microprocessors must be addressed. Semiconductor manufacturers are increasingly replacing the well-known metallization metal aluminum with a metal that allows higher current densities and hence allows reducing the dimensions of the interconnections. For example, copper and alloys thereof are metals generally considered to be a viable candidate for replacing aluminum due to their superior characteristics in view of higher resistance against electromigration and significantly lower electrical resistivity when compared with aluminum.
In spite of these advantages, copper also exhibits a number of disadvantages regarding the processing and handling of copper in a semiconductor facility. For instance, copper may not be efficiently applied onto a substrate in larger amounts by well-established deposition methods, such as chemical vapor deposition (CVD), and copper also may not be effectively patterned by the usually employed anisotropic etch procedures. In manufacturing metallization layers including copper, the so-called damascene technique is therefore preferably used wherein a dielectric layer is first blanket deposited and then patterned to define trenches and vias, which are subsequently filled with copper or copper alloys. A further major drawback of copper is its tendency to readily diffuse in silicon dioxide and other low-k dielectrics.
It is therefore necessary to employ a so-called barrier material in combination with a copper-based metallization to substantially reduce diffusion of copper into the surrounding dielectric material, as copper may readily migrate to sensitive semiconductor areas, thereby significantly changing the characteristics thereof. The barrier material provided between the copper and the dielectric material should, however, in addition to the required barrier characteristics, exhibit good adhesion to the dielectric material as well as to the copper and copper alloys and should also have as low an electrical resistance as possible so as to not unduly compromise the electrical properties of the interconnection as typically the barrier material's electric resistance is significantly greater than the electric resistance of copper and many of the copper alloys. In typical copper-based applications, tantalum and tantalum nitride, alone or in combination, as well as titanium and titanium nitride, alone or in combination, may successfully be employed as barrier layers. However, any other barrier layer schemes may be used as long as the required electrical, diffusion hindering and adhesion characteristics are obtained.
Irrespective of the material used for the barrier layer, with steadily decreasing features sizes, process engineers are increasingly confronted with the challenging task to form respective openings in the dielectric layer and deposit an extremely thin barrier layer within these openings, having significantly high aspect ratios of approximately 5 or more for a trench width or a via diameter of about 0.2 μm and even less. The thickness of the barrier layer has to be chosen as thin as possible to not unduly consume “precious” space of the interconnection that should be filled with the more conductive copper, yet reliably suppressing or preventing the diffusion of the copper into the neighboring dielectric. On the other hand, the etch process for forming the via openings is very critical as, on the one side, the opening has to reliably “land” on, i.e., connect to, the underlying metal or semiconductor region, if a contact opening is considered, while, on the other side, the “consumption” of metal is to be maintained at a low level, when etching into the metal or conductive region, since even after re-filling the via or contact opening, the barrier material may increase the overall resistivity of the underlying metal. Moreover, the deposition of the barrier material within high aspect ratio vias requires improved techniques for physical vapor deposition (PVD) processes or any other processes that are used in depositing conductive materials on a substrate, since usually an enhanced directionality of the barrier atoms and molecules to be deposited is necessary to direct the barrier atoms and molecules to the bottom of the vias and in particular to lower sidewall portions of the vias in order to reliably provide a diffusion barrier for metals such as copper and copper-based materials. On the other hand, the thickness of the barrier layer may be kept at a low level so as to not unduly increase the contact resistance between the metal region or conductive region to the via.
In particular for highly scaled semiconductor devices, a high degree of uniformity of corresponding interconnect structure and contact vias is important, since any variation in resistance and thus current density may lead to fluctuations during operation of the device and may even result in a premature failure of the device.
With reference to FIG. 1, the problems involved in forming vias to underlying metal regions and other conductive regions may be described in more detail. In FIG. 1, a semi-conductor structure 100 comprises a substrate 101, which is to represent any appropriate substrate for the formation of microstructures including conductive and insulating areas, wherein at least some of the conductive areas are used for flowing a current through the semiconductor structure 100. For example, the substrate 101 may comprise a plurality of circuit elements of an integrated circuit, the electrical connection of which may require the formation of one or more “wiring” layers for providing the specified functionality of the integrated circuit. For convenience, any such circuit elements, such as transistors, capacitors and the like, are not shown. Formed above the substrate 101 is a conductive region 102, such as a contact region of a transistor, a capacitor and the like, so that the conductive region 102 may represent a highly doped semiconductor region, a semiconductor region including a metal silicide and the like. In other cases, the conductive region 102 may represent a metal line or any other metal region according to specific design criteria. For example, as previously pointed out, in highly scaled integrated circuits, frequently, copper or copper-based metals are used for forming highly conductive metal regions.
A dielectric layer 103 comprised of any appropriate material or material composition, such as silicon dioxide, silicon nitride, low-k dielectric materials and the like, is formed above the metal region 102, wherein an etch stop layer 106 is typically provided between the metal region 102 and the dielectric layer 103. The etch stop layer 106 may be comprised of any appropriate material that exhibits a high etch selectivity with respect to the material of the dielectric layer 103 to allow an efficient control of an etch process through the dielectric layer 103. For example, silicon nitride, silicon carbide, nitrogen-enriched silicon carbide, silicon dioxide and the like may be appropriate materials for the etch stop layer 106. More-over, a first via 110a is formed on the left hand side of the semiconductor structure 100, wherein the via 110a may be filled with a highly conductive material, such as metal and metal compounds and the like. Typically, a conductive barrier layer 111a may be formed on sidewalls and typically at the bottom of the via 110a to provide an efficient diffusion barrier and also to enhance the mechanical integrity of the via 110a. The via 110a may be formed in accordance with a specific manufacturing sequence, thereby forming the via 110a so that it extends with a certain distance 112a into the conductive region 102. On the right hand side of the semiconductor structure 100 is shown a second via 110b, which may substantially have the same characteristics as the via 110a, which may however represent a via formed in a very different device region or may represent a via formed on a different substrate according to substantially the same manufacturing sequence, wherein, however, slight variations such as process tool drift and the like may result in a slightly different configuration of the via 110b. For example, the barrier layer 111b may exhibit a different thickness and/or the via 110b may extend into the conductive region 102 with a different distance 112b, thereby significantly affecting the overall performance of the semiconductor device 100, since the overall electrical behavior of the conductive region 102 as well as of the vias 110a, 110b may depend on the finally obtained configuration. For example, as previously explained, if a highly conductive material, such as copper or a copper alloy, may be used in the vias 110a, 110b and the conductive region 102, while a significantly less conductive material is used for the barrier layers 111a, 111b, the resulting electrical resistance of the conductive region 102 and also the performance of the vias 110a, 110b may vary according to the magnitude of the distances 112b, 112a. Thus, particularly in highly scaled semiconductor devices, it is important to precisely monitor and control the manufacturing process for the vias 110a, 110b. 
During the formation of the vias 110a, 110b, an anisotropic etch process is performed on the basis of a specified etch recipe, which depends on the material composition of the dielectric layer 103 and other device and process requirements. On the basis of a previously formed resist mask or hard mask, the anisotropic etch process is performed wherein, depending on the manufacturing sequence, vias of different depth or substantially the same depth, as is for instance shown in FIG. 1, have to be formed. Hereby, the anisotropic etch front has to be reliably stopped in and on the etch stop layer 106 to compensate for any across-substrate variations, for different intended etch depths or for substrate-to-substrate variations. In a subsequent etch process, the etch stop layer 106 may be opened, wherein a reliable connection from the via opening into the conductive region 102 is required. Depending on the uniformity of the previous etch process, the etch selectivity of the layer 106, the uniformity of the subsequent etch process for opening the layer 106, and other process non-uniformities, the amount of etching into the conductive region 102 and thus the finally obtained distance 112a, 112b may vary, thereby contributing to the above-explained non-uniformities of the electrical performance of the device 100. After the formation of the via openings within the dielectric layer 103, the etch stop layer 106 and the conductive region 102, the conductive barrier layer 111a, 111b, which may be comprised of one or more materials, may be formed by sputter deposition, chemical vapor deposition (CVD) and the like, wherein typically great efforts are made for forming a very thin layer which, however, reliably covers at least the sidewalls of the via openings down to the bottom, which itself may not necessarily be covered by the respective barrier layer. Thus, in combination with a varying distance 112a, 112b, a variation of the formation process for the barrier layer 111a, 111b may also significantly affect the finally achieved performance of the device 100.
Thus, in conventional monitoring processes, especially the distance 112a, 112b is determined by using SEM (scanning electron microscopy) and/or TEM (transmission electron microscopy) images of cross-sections which, however, requires a great deal of effort in preparing appropriate samples. Moreover, it is a destructive measuring technique and thus provides only low statistics, which may therefore reduce the reliability of the measurement result. Moreover, the entire measurement procedure including the preparation of samples is very slow and thus may limit the ability to provide an efficient process control.
In view of the situation described above, there exists a need for an enhanced monitoring technique which avoids or at least reduces the effects of one or more of the problems identified above.